Add a new NodeFeatures constructor to capture the types of flags

Add a new NodeFeatures constructor to capture the types of flags

When ChannelMessageHandler implementations wish to return a NodeFeatures which
contain all the known flags that are relevant to channel handling, but not
gossip  handling, they currently need to do so by manually constructing a
NodeFeatures with all known flags and then clearing the ones they don't want.

Instead of spreading this logic across the codebase, this consolidates such
construction into one place in features.rs.